Wireless Data Acquisition for a Bridge Collapse Test

"Due to the danger of collapsing a bridge, safety was a primary concern during the test. As such, the WLS-9237 Wireless data acquisition module was used to instrument the load cell that determined the weight of road base applied to the bridge. The wireless system provided the measurement accuracy needed to calculate the collapse load quickly while eliminating wires that would have caused safety issues.
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The Challenge:Finding a quick and safe method to weigh the road fill, applied via a lift bucket on a crane, needed to fail a fracture-critical bridge.

The Solution:Measuring the weight of the road base applied to the bridge by attaching a load cell to the truck-mounted crane load line above the lift bucket and connecting an NI WLS-9237 Wi-Fi data acquisition (DAQ) module to easily read and record the load data from across the work site.

Developing a Wireless Measurement System Using NI Hardware and Software

The Ferguson Structural Engineering Laboratory consists of a structural test facility and a range of loading equipment enabling large-scale studies of structural behavior. In 2009, researchers successfully collapsed a 120 ft bridge with an intentionally fractured girder to study its classification as critical fracture by the American Association of State Highway Transportation Officials. It required three rounds of testing before the damaged bridge finally failed under an applied load of more than 360,000 lb. The bridge, which was tested to determine its vulnerability to collapse following the fracture of a girder, withstood about four and a half times the maximum legal truck load.

The purpose of the tests was to observe the sequence of failure mechanisms and determine the ultimate load required to initiate a total bridge collapse. Due to the system behavior of twin box girder bridges, we needed three rounds of testing, two dynamic and one static. In the first round of testing, we intentionally fractured the bottom flange of one of the girders using an explosive charge. If the bridge was truly fracture-critical, the fracture should have caused the bridge to collapse; however, the bridge survived the first test with no noticeable distress.

We performed another dynamic test to initiate collapse; the bridge was jacked to its original position, the crack in the fractured girder was extended into the webs, and the jack was dynamically removed. Again, the bridge remained intact after this event.

In the final test, the bridge was loaded incrementally until it collapsed. The equivalent of one legal truck was initially placed on the bridge, and load was added in 1,500 to 3,000 lb increments by releasing road fill from a bucket lifted by a crane (Figure 1). More than 100 increments were needed before the bridge finally failed at more than 360,000 lb.

Because of the danger of collapsing a bridge, safety was a primary concern during the test. Thus, we used the NI WLS-9237 Wi-Fi DAQ module to instrument the load cell that determined the weight of road base applied to the bridge. The wireless system provided the measurement accuracy needed to calculate the collapse load quickly while eliminating wires that would have caused safety issues. The system allowed for simple but secure IEEE 802.11g wireless direct sensor connectivity.

We used LabVIEW software to remotely monitor the load signals. With built-in signal conditioning and the highest commercially available network security, this made it possible to stream data in real time to a remote monitoring location that was located approximately 50 ft away (Figure 2).

Test Procedure

After placing concrete girders that formed a bin on the bridge deck to simulate the weight of a truck, we applied an additional load by incrementally dumping material onto the bridge (Figure 3). The road base was chosen as the loading material for its ease of acquisition, low cost, and relatively high density.

To measure the weight quickly and safely, we attached a load cell to the crane load line above the lift bucket. We also built a small wooden box to house the wireless transmitter and its power supply (Figure 4), which was hung on the crane near the load cell. We used a simple laptop battery to power the WLS-9237. Lastly, we connected a steel sling between the load cell and the lift bucket to provide sufficient clearance so that the road base dumped into the bucket would not damage the load cell or associated equipment. The WLS-9237 Wi-Fi DAQ device was connected to the load cell so that the load data could be easily read and recorded from across the work site and not interfere with the operation of the crane.

Bridge Data Acquisition System

In addition to specification and executing the full-scale test safely, it was important that data was acquired during the experiment for future bridge behavior analysis. We designed and implemented an instrumentation plan to measure deflections and material strains. Strain gages attached directly to bridge components took measurements of material deformations.

National Instruments manufactured all of the hardware, including the DAQ system that had 244 channels (Figure 5). Two NI SCXI-1001 12-slot chassis were filled to capacity with a total of 24 NI SCXI-1520 8-channel universal strain modules. We also used two additional NI SCXI-1000 4-slot chassis, and five additional NI SCXI-1520 8-channel universal strain modules and three NI SCXI-1121 4-channel isolation amplifiers to fill the eight new slots. NI SCXI-1314 8-channel terminal blocks were connected to each of the 29 NI SCXI-1520 modules, and three NI SCXI-1321 4-channel terminal blocks were connected to the NI SCXI-1121 isolation amplifiers. All four of the chassis were connected through the NI PCI-6250 DAQ board in the PC, which we configured with LabVIEW.

Testing to Failure

After three days of loading the bridge with road fill, large portions of concrete at the mid-span expansion joint of the exterior railing began to spall when the total load applied to the bridge reached approximately 360,000 lb. After the onset of major material losses, three additional lift bucket loads were placed on the bridge before it came to rest on the concrete bed (Figure 6). As the load applied to the bridge increased over the course of the experiment, the bridge components experienced a series of failures. Following these intermediate failures, the bridge was able to redistribute the applied loads, suggesting the contribution of redundant load paths in maintaining equilibrium of the bridge in its progressively damaged state. We will use these findings to develop strength models to evaluate twin box girder bridges.

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